Passive cooling system for auxiliary power unit installation

ABSTRACT

A passive cooling system for an auxiliary power unit (APU) installation on an aircraft is provided. The system is for an auxiliary power unit having at least a compressor portion of a gas turbine engine and an oil cooler contained separately within a nacelle. The system includes the auxiliary power unit housed within the nacelle of the aircraft, an engine exhaust opening defined in the aft portion of the nacelle and communicating with the gas turbine engine, at least a first air inlet duct communicating with a second opening defined in said nacelle and with said compressor portion and the oil cooler is located within a second duct communicating with an opening other than the engine exhaust opening of said nacelle and with the engine exhaust opening. Exterior cooling air and engine exhaust ejected through said engine exhaust opening entrain cooling air through said second duct to said oil cooler, and thus provide engine oil cooling. An exhaust eductor is also provided.

TECHNICAL FIELD

[0001] The present invention relates to cooling systems for auxiliarypower units on airplanes and, more particularly, pertains to the passivecooling of the components and oil of such units and the enclosureventilation of such units.

BACKGROUND OF THE INVENTION

[0002] Large aircraft often use an on-board auxiliary power unit (APU)to provide electrical power and compressed air for systems throughoutthe airplane. When the aircraft is grounded, the APU provides the mainsource of power for environmental control systems, hydraulic pumps,electrical systems and main engine starters. During flight, the APU cansupply pneumatic and electric power.

[0003] Auxiliary power units are generally small gas turbine engines,often mounted in the aft tail section of the aircraft. They require acertain amount of cooling air, and are lubricated by oil that isgenerally cooled by an oil cooler which also requires cooling air.Active cooling systems are usually employed to provide this cooling air,and are typically comprised of an active fan used to push air throughthe oil cooler and across auxiliary power unit components. These fansare driven at high speeds by the APU through a complex shaft and gearassembly. The mechanical complexity and high operating speeds of thesefans increases the possibility of failure. Active fan cooling systemstherefore can significantly reduce the reliability of an auxiliary powerunit.

[0004] While APU passive cooling systems which eliminate the need foractive fan cooling systems are well known, they all generally drawcooling air into the APU compartment, before it is drawn through the aircooled oil cooler. This arrangement causes the cooling air to be heatedup in the compartment before it reaches the oil cooler, and therefore,oil cooling is not optimized. U.S. Pat. No. 5,265,408, for example,discloses a method and apparatus for cooling a compartment mounted gasturbine engine comprising a first exhaust eductor within which ismounted an oil cooler, and which incorporates a mixer nozzle to entraincooling air flow first through the APU compartment and then through theoil cooler. Surge bleed flow from the load compressor is discharged intothe exhaust eductor. Ambient air is received into the compartmentthrough a second exterior eductor inlet.

[0005] U.S. Pat. No. 5,655,359 similarly discloses an APU passivecooling system wherein cooling air for the oil cooler is drawn from thecompartment. An inlet scoop in the engine air intake duct used to diverta portion of the air flow into the APU compartment. This air is used tocool the engine before being drawn through the oil cooler, mounted in avacuum duct, by a lobed mixer which acts as an aspirator.

[0006] U.S. Pat. No. 6,092,360 discloses an APU passive cooling systemin which cooling air is drawn into the engine compartment through anopening located in the rear of the aircraft. An eductor mounted beforethe exhaust duct of the engine, draws compartment air through the oilcooler, which in turn draws atmospheric air in through the aft opening.

[0007] Thus, while these patents provide for cooling of an auxiliarypower unit without the use of a mechanically driven fan, they all teachsystems which draw cooling air for the oil cooler from the APUcompartment. A need exists for an auxiliary power unit passive coolingsystem that can provide enhanced oil cooling capabilities by directingexterior cooling air, through ducts, directly to the oil cooler, andwhich is nevertheless adaptable enough to be able to provide damageprotection from foreign objects and be combined with the enginecompressor surge bleed flow to provide improved airflow through the oilcooling heat exchanger.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide an improvedcooling system for an auxiliary power unit on an airplane.

[0009] It is also an object of the present invention to provide asimpler cooling system for auxiliary power unit engine oil and externalcomponents which does not require moving parts and does not include acooling fan.

[0010] It is a further object of the present invention to provideimproved cooling of the oil in an auxiliary power unit by providingenhanced cooling airflow through the heat exchanger.

[0011] Therefore, in accordance with the present invention there isprovided a passive cooling system for an auxiliary power unitinstallation on an aircraft, comprising an auxiliary power unit housedwithin a nacelle of the aircraft, the auxiliary power unit comprising atleast a compressor portion of a gas turbine engine and an oil coolercontained separately within the nacelle, an engine exhaust openingdefined in the aft portion of the nacelle and communicating with the gasturbine engine, at least a first cooling air inlet duct communicatingwith a second opening defined in the nacelle and with the compressorportion, the oil cooler located within a second duct communicating withthe exterior of the nacelle and the engine exhaust opening wherebyexterior cooling air, and engine exhaust ejected through the engineexhaust opening entrains cooling air through the second duct to the oilcooler, providing engine oil cooling.

[0012] In accordance with the present invention, there is also provideda passive cooling system for an auxiliary power unit installation on anaircraft, comprising: an auxiliary power unit housed within a nacelle ofthe aircraft, the auxiliary power unit comprising at least a compressorportion of a gas turbine engine and an oil cooler contained separatelywithin said nacelle; an engine exhaust opening defined in the aftportion of said nacelle and communicating with said gas turbine enginevia an exhaust eductor assembly; said exhaust eductor assembly being influid flow communication with a compressor surge bleed duct; at least afirst air inlet duct communicating with a second opening defined in saidnacelle and with said compressor portion; and said oil cooler locatedwithin a second duct communicating with an opening other than the engineexhaust opening of said nacelle and with said engine exhaust opening,whereby exterior cooling air and engine exhaust ejected through saidexhaust eductor assembly, entrain cooling air through said second ductto said oil cooler, providing engine oil cooling.

[0013] In accordance with a more specific embodiment of the presentinvention, the engine air inlet includes a first duct portion, and thesecond duct is bifurcated from the first duct portion and extendsdownstream from the first duct portion with a third duct portion alsoformed downstream of the first duct, the third duct portioncommunicating with the compressor portion and the oil cooler locatedwithin the second duct portion providing direct exterior cooling air tothe oil cooler.

[0014] In one embodiment, contamination of aircraft environmentalcontrol system air is prevented by an air inlet splitter, which isolatesthe load compressor gas path. Protection against damage from foreignobjects, for the powerplant, may be provided by a bypass duct locatedin-line with the first air inlet duct, and a scavenger discharge ductand outlet which expels harmful foreign objects from the aircraft. Thenacelle is provided with a rear exhaust opening, and at least a secondopening for the outside air intake. The third air inlet duct portiondirects the air from the air intake to the engine compressor portion.The auxiliary power unit comprises a gas turbine engine having both loadand core compressors and a compressor surge bleed valve and duct. Theoil cooler may comprise an air-to-oil heat exchanger. The engine exhaustejector creates a depressurization in the nacelle or in the exhausteductor assembly, which results in the entrainment of cooling airthrough the heat exchanger and through the nacelle. In at least oneembodiment, a dedicated small opening in the exhaust eductor assemblypermits nacelle ventilation.

[0015] Further features and advantages of the present invention willbecome fully apparent by referring to the following detaileddescription, claims, and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a cross sectional schematic illustration of a firstembodiment of the APU passive cooling system in accordance with thepresent invention.

[0017]FIG. 2a is a cross sectional schematic illustration of a secondembodiment of the APU passive cooling system in accordance with thepresent invention.

[0018]FIG. 2b is a cross sectional schematic illustration of the secondembodiment of the APU passive cooling system shown in FIG. 2a.

[0019]FIG. 3 is a cross sectional schematic illustration of a thirdembodiment of the APU passive cooling system in accordance with thepresent invention.

[0020]FIGS. 4a to 4 d are cross sectional schematic illustrations of afourth embodiment of the APU passive cooling system in accordance withthe present invention.

[0021]FIG. 5 is a perspective view of an engine having a main air inletduct and exhaust eductor assembly in accordance with the presentinvention.

[0022]FIG. 6 is a vertical cross-sectional view of an exhaust eductorassembly used in accordance with the present invention.

[0023]FIG. 7 is a perspective view of the cooling air flow inner shroudof the exhaust eductor assembly shown in FIG. 6.

[0024]FIG. 8 is a side perspective view of the exhaust eductor assemblyshown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] Referring to the drawings, FIG. 1 embodies an APU installation 10comprising the elements of the present invention that will be described.The APU installation 10 is principally comprised of a gas turbine powerplant 12 and an oil cooler 14, both within an auxiliary power unitnacelle 16. This nacelle is defined for the purposes of the presentinvention, as any dedicated enclosed compartment or enclosure, generallyalthough not essentially located within the aircraft tailcone. Thenacelle 16 shown in these embodiments as an aft compartment in theaircraft, has an exterior skin surface 18. Compartment access doors 42allow external access to the auxiliary power unit when the aircraft ison the ground, for such purposes as engine maintenance.

[0026] In the embodiment shown in FIG. 1, the exterior surface 18 of theAPU nacelle 16 comprises principally two openings, the rear exhaustopening 20 and the main air inlet opening 22. The main air inlet opening22 in the aircraft exterior skin 18 allows air to be drawn from outsidethe aircraft by the power plant compressors. The gas turbine enginepower plant 12 is comprised of two compressors, a load compressor 34 anda core compressor 36. The load compressor 34 provides the aircraftenvironmental control system (ECS) air, while the core compressor 36provides the powerplant with air for combustion. Inlet air is directedby a first air inlet duct 24 from the air inlet opening 22 to the powerplant compressors. The oil cooler 14, shown to be normal to thedirection of airflow but is not necessarily limited to this orientation,is located in a second duct 27. A bifurcation 26 in the first air inletduct 24 is provided to directly supply cooling air to the oil cooler 14,in the form of an air-to-oil heat exchanger, through the second duct 27.This allows air to be directly fed to the oil cooler 14 through a duct,providing improved cooling airflow. After passing through the oilcooler, the cooling air enters the APU compartment 16 to provide coolingto the APU components.

[0027] An exhaust ejector 38 of the powerplant 12, causes adepressurization of the APU compartment 16. The exhaust ejector 38achieves this by reducing the diameter of the power plant exhaustpassage, causing an increase in the velocity of the exhaust gases. Thiscauses the depressurization upstream in the APU compartment 16,resulting in entrainment of the cooling air through the heat exchangerand the APU compartment, thereby cooling the engine oil and thepowerplant components within the APU compartment.

[0028] Within the first air inlet duct 24 is located an air inletsplitter 28. The splitter 28 in the engine air inlet duct 24 extendsdown into the engine intake plenum 30. The air splitter 28 and thebifurcation 26 in the first air inlet duct are positioned such that thebifurcation 26 in the inlet duct is downstream of the leading edge 32 ofthe splitter 28. When the power plant is run with the access doors 42open, the resulting ambient pressure in the APU compartment 16 equalizeswith the outside air pressure, which causes a flow reversal within theheat exchanger as the power plant creates a depression within the firstair inlet duct 24. In this operating mode, a reversal of airflow occurs,as the air is entrained from the compartment, through the heat exchangerand the second duct 27, and gets ingested into the engine. The splitter28, consequently, prevents contamination of the airflow of the loadcompressor 34 in the event of a leak in the heat exchanger 14 when thepowerplant is operated with the compartment access doors 42 open.Therefore, any oil leaked from the heat exchanger is forced down thecore compressor and burned by the engine, rather than contaminating theaircraft environmental control system air.

[0029]FIGS. 2a and 2 b illustrate an alternate embodiment of the passivecooling system. Referring to the embodiment illustrated in FIG. 2a, aheat exchanger air inlet duct 59 directs cooling air from a bifurcation58 in the main engine air inlet duct 24 to the heat exchanger 14, and aheat exchanger discharge duct 52 directs cooling air downstream of theheat exchanger 14 directly to an exhaust eductor assembly 57. Theexhaust eductor, or exhaust ejector plenum, while it is generally anannular plenum adapted to receive exiting APU cooling air which is drawnthrough the eductor and into the engine exhaust by the depressurizationcaused by the engine exhaust ejector 38, could alternately be anysimilar device of varying shape which performs the equivalent function.APU component cooling air is admitted into the APU compartment through asmall second bifurcation 54 in the heat exchanger air inlet duct 59. Thecomponent cooling air then exits the APU compartment 16 through anothersmall bifurcation 56 in the exhaust eductor assembly 57. The surge bleedduct 48 is combined with the heat exchanger discharge duct 52 downstreamof both the surge bleed valve 50 and the heat exchanger 14. Thiscombined heat exchanger and surge bleed duct design, while preventingcontaminating oil from the heat exchanger 14 from entering the aircraftbleed system or the ECS air, provides further enhanced airflow throughthe heat exchanger when the surge bleed valve 50 is open.

[0030] In the embodiments shown in FIGS. 2a and 2 b, the oil cooler islocated further forward with respect to the engine, nearer the gearboxcasing of the power plant and close to the oil pumps of the engine. Thiseliminates the need for long oil lines. The surge bleed valve 50 isclosed when the APU supplies bleed air to the aircraft. However, whenthe APU only supplies electric power, the surge bleed valve 50 isopened, and the junction between the surge bleed duct 48 and the heatexchanger discharge duct 52 is designed to enhance air flow through theheat exchanger using the additional kinetic energy of the surge bleedflow, thereby improving oil cooling. As in the embodiment of FIG. 1, theexhaust ejector 38, here within the exhaust eductor assembly 57, causesthe entrainment of cooling air flow through the heat exchanger and outthrough the engine exhaust duct.

[0031]FIG. 2b illustrates a similar embodiment as FIG. 2a, having,however, a dedicated heat exchanger opening 44 in the exterior surface18 of the nacelle compartment 16. This provides outside air via thealternate heat exchanger inlet duct 46 to the heat exchanger 14. In thisembodiment, the compartment cooling air inlet 54 is shown to be locatedin the first air inlet duct 24 rather than the heat exchanger inlet duct59. Nevertheless, either location for the compartment air inlet 54 ispossible. The embodiment shown in FIG. 2b, however, provides independentair cooling sources for the oil cooler and the engine components withinthe APU compartment. The advantage of this embodiment over that shown inFIG. 1 is that more efficient cooling of the engine components isachieved because cooling air does not first get warmed by first goingthrough the heat exchanger before it reaches the APU components.

[0032]FIG. 3 shows a further embodiment of the present invention. Thisembodiment additionally includes a duct providing foreign object damageprotection for the engine. The power plant compressors draw air from theoutside through a main air inlet opening 22 in the aircraft skinexterior surface 18. The engine air inlet duct 24 directs the air to theengine compressors. According to the embodiment shown in FIG. 2b, asmall bifurcation 54 in the inlet duct is provided to supply cooling airto the APU compartment. The exhaust ejector 38 within the exhausteductor assembly 57 creates a depressurization of the APU compartmentresulting in airflow through the bifurcation opening 54 in the air inletduct. Cooling air exits the APU compartment through a second bifurcation56 in the exhaust eductor assembly 57.

[0033] An in-line bypass duct 60 is adjoined to the first air inlet duct24, in order to direct cooling air to the heat exchanger 14, located inthe mouth of the eductor assembly 57 parallel to the direction ofairflow in the bypass duct. The airflow in the bypass duct 60 issustained by the eductor induced flow through the oil cooling heatexchanger. One advantage this embodiment permits is the use of a smalleroil cooler. A scavenge discharge duct 62 is used as a collector todischarge overboard any foreign objects collected by the bypass duct 60.The bypass and scavenge ducts are designed such that separated liquidand solid particles will drain or be drawn by gravity out through thescavenge duct exit 64. The scavenge duct 62 and scavenge exit 64 aresized such that flow reversal is minimized during aircraft static andlow speed conditions which cause flow reversal in the scavenge duct. Theair bypass and the scavenge ducts 60 and 64 respectively, provide alevel of foreign object damage protection for the powerplant.

[0034]FIGS. 4a to 4 d show another embodiment of the present inventionwherein the oil cooler 14 is located within the exhaust eductor assembly57 and the dedicated heat exchanger inlet duct 46 feeds air directlyfrom the aircraft exterior to the oil cooler. Dedicated heat exchangeropening 44 in the exterior surface 18 of the aircraft's nacellecompartment 16 permits exterior air to be fed through the inlet duct 46to the oil cooler 14 located perpendicular to the inlet air flow in theannular exhaust eductor assembly 57. The engine exhaust ejector 38within the eductor assembly 57 draws the cooling air through the heatexchanger inlet duct 46 and the oil cooler 14, and out into the mainengine exhaust duct 40.

[0035] The variations of the fourth embodiment of the present inventionshown in FIGS. 4a to 4 d, involve alternate locations of the compartmentcooling air inlet and exits. FIG. 4a shows an embodiment wherein thecompartment cooling air inlet 54 is a bifurcation in the main engine airinlet duct 24. This permits air to enter the nacelle compartment 16 toprovide cooling to the externals of the APU. This cooling air then exitsthe compartment through a bifurcation in the heat exchanger inlet duct46 for the compartment cooling air exit 68. The embodiment shown in FIG.4b uses a compartment cooling air inlet 70 in the heat exchanger inletduct 46. The compartment cooling air then exits the nacelle compartmentthrough a small bifurcation 56 in the exhaust eductor assembly 57,similar to the embodiments of FIGS. 2 and 3. The embodiments of FIGS. 4cand 4 d both have a separate compartment cooling air inlet 72 in theexterior surface 18 of the nacelle compartment 16. The engine exhaustejector 38 pulls cooling air from the exterior of the aircraft via theair inlet 72, through the compartment 16, and out through either the airexit bifurcation 56 in the exhaust eductor 57, as shown in FIG. 4c, orthe air exit bifurcation 68 in the heat exchanger inlet duct 46, asshown in FIG. 4d.

[0036]FIG. 5 shows an embodiment of the APU installation 10, comprisingthe gas turbine power plant 12, the oil cooler 14 and the exhausteductor assembly 57.

[0037] The assembly shown in FIG. 6 consists of a construction of sheetmetal components either welded or riveted together. The assembly is ofmodular design and is supported by the engine exhaust casing 81. Theexhaust eductor assembly comprises a primary nozzle 82 locatedimmediately downstream of the engine exhaust gas path. The gas path ofthe primary nozzle 82 is bounded by the primary nozzle shroud 83 andexhaust plug 84. The primary nozzle is circumscribed by the surge bleednozzle 85, which is bounded by the cooling air plenum inner shroud 86and the primary nozzle shroud 83. The cooling air mixing plane islocated downstream of the primary nozzle 82.

[0038] Mixing lobes 87 are introduced to improve the mixing efficiency,thereby resulting in improved cooling mass flow. The number of lobeswithin the eductor assembly inner shroud may vary depending on exhaustduct diameter and cooling air flow requirements. Similarly, thegeometrical shape of the mixing lobes 87 may vary based on pumpingrequirements and acoustics. These mixing lobes 87 can be either weldedor mechanically fastened to the cooling air plenum inner shroud 86.

[0039] The eductor assembly incorporates a primary surge bleed plenum 88in which the surge bleed flow is redistributed circumferentially beforeexiting through a series of openings on the surge bleed flow plenuminner shroud 89 and entering the secondary surge bleed plenum 90. Inthis plenum, the surge bleed flow is realigned axially and then ejectedback into the main engine gas path through the surge bleed nozzle 85.The primary surge bleed plenum 88 is fed, during specific engineoperating conditions, by the surge bleed duct 48. This surge bleed flowis controlled by the modulating surge bleed valve 50 located in thesurge bleed duct 48. Flow from the surge bleed duct 48 enters theprimary surge bleed plenum 88, at the junction 93 of the two components,in a radial direction and impinges directly on the diaphragm 94, whichdivides the primary surge bleed plenum 88 and the cooling air plenum 95.This diaphragm 94 has a conical shape and acts as a natural splitter toredistribute the surge bleed flow uniformly around the circumference ofthe surge bleed plenum inner shroud 89.

[0040] The cooling air plenum 95 located on the aft side of thediaphragm 94 is bounded by the cooling air plenum outer shroud 96 andinner shroud 86. Openings 97 are provided on the outer shroud for thecooling air to enter the cooling air plenum 95. The air cooled heatexchanger 14 is located upstream of these openings. Both the surge bleedflow and the cooling air flow plenums 88 and 95 respectively are sealedto prevent any leakage.

[0041] A mechanical interface 98 is provided on the downstream end ofthe eductor assembly for connecting to the aircraft exhaust duct 40.Opening 56 is provided on the cooling air plenum outer shroud in orderto accept ventilation air exiting from the engine compartment. Thecutouts 80 on the cooling air flow inner shroud 86, as seen in FIG. 7,are provided in line with each mixing lobe 87.

[0042] The layout of the eductor assembly as described in detail aboveoffers several additional advantages. The engine exhaust velocity can beeasily altered by changing a simple axisymmetric part, namely, theprimary nozzle shroud 83, in order to improve the amount of secondaryair flow used for cooling purposes. This can be easily done withoutrequiring modification of any of the more complex and more expensiveparts of the eductor assembly. Also, a large exhaust plug 84 is requiredin order to control the air flow in the primary nozzle 82 and the airflow into the primary passages of the mixing lobes 87. The resultinglarge volume of space inside the exhaust plug 84 can then be used foracoustic treatment, for example, by introducing inside the plug lowfrequency cavities extending from the engine exhaust casing 81 interfaceto the cooling air flow mixing plane.

[0043] Therefore, in summary, the eductor assembly and passive coolingsystem of the present invention, provides engine oil cooling and engineenclosure cooling without requiring the use of any rotating parts andpermits the reinjection of surge bleed flow into the main engine exhaustgas path thereby providing additional pumping capability to the coolingair. The eductor assembly is additionally capable of redistributing thesurge bleed flow circumferentially within the surge bleed plenum,providing a method for controlling the pumping capability of the eductorassembly by the introduction of a simple axisymmetric primary nozzleshroud into the main exhaust gas path, and providing a method to controlthe noise generated by the engine in the eductor assembly by theintroduction of a large exhaust plug with internal acoustic chambers.

[0044] The embodiments of the invention described above are intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

What is claimed is:
 1. A passive cooling system for an auxiliary powerunit installation on an aircraft, comprising: an auxiliary power unithoused within a nacelle of the aircraft, the auxiliary power unitcomprising at least a compressor portion of a gas turbine engine and anoil cooler contained separately within said nacelle; an engine exhaustopening defined in the aft portion of said nacelle and communicatingwith said gas turbine engine; at least a first air inlet ductcommunicating with a second opening defined in said nacelle and withsaid compressor portion; and said oil cooler located within a secondduct communicating with an opening other than the engine exhaust openingof said nacelle and with said engine exhaust opening, whereby exteriorcooling air and engine exhaust ejected through said engine exhaustopening, entrain cooling air through said second duct to said oilcooler, providing engine oil cooling.
 2. The passive cooling system asdefined in claim 1, wherein said second duct is bifurcated from saidfirst air inlet duct, and extends downstream from said first ductportion, and a third duct portion, also formed downstream of said firstduct portion, communicates with said compressor portion.
 3. The passivecooling system as defined in claim 1, wherein said second duct iscommunicating with a third opening defined in said nacelle and with saidengine exhaust opening.
 4. The passive cooling system as defined inclaim 1, wherein said compressor portion comprises a load compressor anda core compressor.
 5. The passive cooling system as defined in claim 1,wherein an opening defined in one of said first duct and said secondduct, is in communication with the exterior of said gas turbine enginewithin said nacelle.
 6. The passive cooling system as defined in claim2, wherein said first duct comprises an airflow splitter with a leadingedge upstream of the bifurcation of said second duct, and said secondduct empties into said nacelle downstream of said oil cooler.
 7. Thepassive cooling system as defined in claim 1, wherein said oil coolercomprises an air to oil heat exchanger.
 8. The passive cooling system asdefined in claim 1, wherein said engine exhaust opening is in fluid flowcommunication with an exhaust eductor assembly.
 9. The passive coolingsystem as defined in claim 1, wherein said nacelle is located within thetailcone of the aircraft.
 10. The passive cooling system as defined inclaim 1, wherein said nacelle has external access doors.
 11. The passivecooling system as defined in claim 1, wherein said second duct isintegrated with a compressor surge bleed duct, downstream of said oilcooler.
 12. The passive cooling system as defined in claim 8, whereinsaid exhaust eductor assembly comprises a dedicated opening for the exitof cooling air from said nacelle.
 13. The passive cooling system asdefined in claim 8, wherein protection for said gas turbine engine fromforeign object damage is provided.
 14. The passive cooling system asdefined in claim 13, wherein said second duct is in direct communicationwith said exhaust eductor, said oil cooler is located at the junction ofsaid second duct and said exhaust eductor, and said oil cooler isoriented parallel to and offset from the airflow through said secondduct.
 15. The passive cooling system as defined in claim 14, wherein ascavenge discharge duct is in fluid flow communication with said secondduct and with a further opening defined in said nacelle.
 16. The passivecooling system as defined in claim 8, wherein said second duct isdirectly communicating with a third opening defined in said nacelle andwith said exhaust eductor assembly, said oil cooler is located at thejunction of said second duct and said exhaust eductor assembly, and saidoil cooler is oriented perpendicular to the airflow through said secondduct.
 17. The passive cooling system as defined in claim 16, whereinsaid second duct comprises a dedicated opening for one of an inlet andan exit of cooling air for said nacelle.
 18. The passive cooling systemas defined in claim 8, wherein a further opening defined in said nacelleis in communication with said exhaust eductor assembly.
 19. The passivecooling system as defined in claim 18, wherein said further opening isan air inlet to said nacelle and said exhaust eductor provides an airexit from said nacelle.
 20. The passive cooling system as defined inclaim 19, wherein said further opening and said exhaust eductor are incommunication via one of a dedicated opening in said exhaust eductor anda dedicated opening in said second duct.
 21. The passive cooling systemas defined in claim 8, wherein said exhaust eductor assembly is in fluidflow communication with a compressor surge bleed duct.
 22. The passivecooling system as defined in claim 8, wherein said exhaust eductorassembly is in direct fluid flow communication with said second duct.23. The passive cooling system as defined in claim 22, wherein mixingnozzles within said exhaust eductor assembly integrate said cooling airfrom said second duct with said engine exhaust.
 24. The passive coolingsystem as defined in claim 23, wherein said exhaust eductor assemblycomprises an axisymmetric primary nozzle located upstream of said mixingnozzles.
 25. The passive cooling system as defined in claim 24, whereinsaid axisymmetric primary nozzle defines a velocity of said engineexhaust, and correspondingly a volume of said cooling air entrainedthrough said second duct.
 26. The passive cooling system as defined inclaim 24, wherein said axisymmetric primary nozzle is defined by anouter annular shroud and a central exhaust plug.
 27. The passive coolingsystem as defined in claim 26, wherein said central exhaust plugcomprises cavities therein for providing acoustic treatment.
 28. Thepassive cooling system as defined in claim 27, wherein said cavitiesattenuate low frequency sounds.
 29. A passive cooling system for anauxiliary power unit installation on an aircraft, comprising: anauxiliary power unit housed within a nacelle of the aircraft, theauxiliary power unit comprising at least a compressor portion of a gasturbine engine and an oil cooler contained separately within saidnacelle; an engine exhaust opening defined in the aft portion of saidnacelle and communicating with said gas turbine engine via an exhausteductor assembly; said exhaust eductor assembly being in fluid flowcommunication with a compressor surge bleed duct; at least a first airinlet duct communicating with a second opening defined in said nacelleand with said compressor portion; and said oil cooler located within asecond duct communicating with an opening other than the engine exhaustopening of said nacelle and with said engine exhaust opening, wherebyexterior cooling air and engine exhaust ejected through said exhausteductor assembly, entrain cooling air through said second duct to saidoil cooler, providing engine oil cooling.
 30. The passive cooling systemas defined in claim 29, wherein said exhaust eductor assembly comprisesan annular axisymmetric primary nozzle upstream of a plurality ofradially located mixing nozzles.
 31. The passive cooling system asdefined in claim 30, wherein said mixing nozzles integrate said coolingair from said second duct with said engine exhaust.
 32. The passivecooling system as defined in claim 31, wherein said annular axisymmetricprimary nozzle defines a velocity of said engine exhaust, andcorrespondingly a volume of said cooling air entrained through saidsecond duct.
 33. The passive cooling system as defined in claim 32,wherein said annular axisymmetric primary nozzle is defined by outerannular shroud and a central exhaust plug.
 34. The passive coolingsystem as defined in claim 33, wherein said central exhaust plugcomprises internal cavities adapted to provide acoustic attenuation.